Question 13 Marks
The first term of a G.P. is 1. The sum of the third term and fifth term is 90. Find the common ratio of G.P.
AnswerGiven: $a = 1$ and $a_3 + a_5 = 90$
$\Rightarrow ar^2 + ar^4 = 90$
$\Rightarrow a(r^2 + r4) = 90$
$\Rightarrow 1 \times \left( r ^ { 2 } + r ^ { 4 } \right) = 90$$\Rightarrow r^2 + r^4 = 90$
$\Rightarrow r^4 + r^2 - 90 = 0$
$\Rightarrow r ^ { 2 } = \frac { - 1 \pm \sqrt { ( 1 ) ^ { 2 } - 4 \times ( - 90 ) \times 1 } } { 2 \times 1 }$
$= \frac { - 1 \pm \sqrt { 1 + 360 } } { 2 } = \frac { - 1 \pm \sqrt { 361 } } { 2 }$
$= \frac { - 1 \pm 19 } { 2 }$=
$\Rightarrow r ^ { 2 } = \frac { - 1 + 19 } { 2 } = \frac { 18 } { 2 } = 9$or $r ^ { 2 } = \frac { - 1 - 19 } { 2 } = \frac { - 20 } { 2 } = - 10$ which is not possible
Therefore, the common ratio is $r = \pm 3$
View full question & answer→Question 23 Marks
The sum of some terms of G.P. is 315 whose first term and the common ratio are 5 and 2 respectively. Find the last term and the number of terms.
AnswerGiven: $a=15, r=2$ and $\mathrm{S}_{\mathrm{n}}=315$
$\therefore \mathrm{S}_n=\frac{a\left(r^n-1\right)}{r-1}$
$\Rightarrow 315=\frac{5\left(2^n-1\right)}{2-1}$
$\Rightarrow \frac{315}{5}=2^n-1$
$\Rightarrow 2^{n-1}=63$
$\Rightarrow 2^n=64=2^6$
$\Rightarrow n=6$
$\therefore a_6=a r^{6-1}=5 \times 2^5=5 \times 32=160$
Hence the number of terms $=6$ and the last term $=160$
View full question & answer→Question 33 Marks
If f is a function satisfying f (x+y) = f (x) f (y) for all x, y $ \in $ N such that f (1) = 3 and $\sum\limits_{x = 1}^n f (x) = 120$ find the value of n.
Answer$f(1)=3 f(1+2)=f(1) f(2)=3 \times 9=27$
$f(1+3)=f(1) f(3)=3 \times 27=81$
L.H.S.
$=f(1)+f(2)+f(3)+\ldots \ldots .+f(n)$
$=3+9+27+81+\ldots+n \text { terms }$
$=\frac{3\left(3^n-1\right)}{3-1}=\frac{3}{2}\left(3^n-1\right)$
$=f(1)+f(2)+f(3)+\ldots-+f(n)$
$=3+9+27+1+\ldots-+n \text { terms }$
$=\frac{3\left(3^n-1\right)}{3-1}=\frac{3}{2}\left(3^n-1\right)$
ATQ
$\frac{3}{2}\left(3^n-1\right)=120$
$3^n-1=80$
$3^n=81$
$n=4$
View full question & answer→Question 43 Marks
Find the sum of all two digit numbers which when divided by 4, yield 1 as remainder.
AnswerGiven: A.P. 13, 17, 21, .... , 97
Here a = 13, d = 17 - 13 = 4 and $a_n = 97$
Now, $a_n = a + (n - 1)d$
$\Rightarrow 97 = 13 + ( n - 1 ) \times 4$
$\Rightarrow 97 - 13 = ( n - 1 ) \times 4$
$\Rightarrow 84 = ( n - 1 ) \times 4$
$\Rightarrow n - 1 = \frac { 84 } { 4 } = 21$
$\Rightarrow$ n = 22
$\therefore$ $S _ { n } = \frac { n } { 2 } \left( a + a _ { n } \right)$
$\Rightarrow \mathrm { S } _ { 22 } = \frac { 22 } { 2 } ( 13 + 97 )$
$\Rightarrow \mathrm { S } _ { 22 } = \frac { 22 } { 2 } \times 110=1210$
View full question & answer→Question 53 Marks
Find the sum of integers from 1 to 100 that are divisible by 2 or 5.
AnswerDivisible by 2
$2,4,6, \ldots 100$
$a=2, d=2, a_n=100$
$100=2+(n-1) \times 2$
$n=50$
$S_{50}=\frac{50}{2}[2+100]=2550$
divisible by 5
$a=5, d=5, a_n=100$
$5+(n-1) \times 5=100$
$n=20$
$S_{20}=1050$
divisible by both 2 or 5
$10,20,30, \ldots \ldots .100$
$a=10, \mathrm{~d}=10, \mathrm{a}_{\mathrm{n}}=100$
$100=10+(\mathrm{n}-1) \times 10$
$\mathrm{n}=10$
$S_{10}=\frac{10}{2}[10+100]$
$=550$
According to question, Sum $=2550+1050-550$
$=3050$
View full question & answer→Question 63 Marks
Find the sum of all numbers between 200 and 400 which are divisible by 7.
AnswerGiven: A.P. 203, 210, 217, ………., 399
Here a = 203, d = 210 - 203 = 7 and $a_n = 399$
Now, $a_n = a + (n - 1)d$
$\Rightarrow 399 = 203 + ( n - 1 ) \times 7$
$\Rightarrow 399 - 203 = ( n - 1 ) \times 7$
$\Rightarrow 196 = ( n - 1 ) \times 7$
$\Rightarrow n - 1 = \frac { 196 } { 7 } = 28$
$\Rightarrow$ n = 29
$\therefore$ $S _ { n } = \frac { n } { 2 } \left( a + a _ { n } \right)$
$\Rightarrow S _ { 29 } = \frac { 29 } { 2 } ( 203 + 399 )$
$\Rightarrow \mathrm { S } _ { 29 } = \frac { 29 } { 2 } \times 602=8729$
View full question & answer→Question 73 Marks
150 workers were engaged to finish a job in a certain number of days 4 workers dropped out on the second day, 4 more workers dropped out on the third day and so on. It took 8 more days to finish the work. Find the number of days in which the work was completed.
AnswerGiven, a = 150, d = -4
$S _ { n } = \frac { n } { 2 } [ 2 \times 150 + ( n - 1 ) ( - 4 ) ]$
If total works who would have worked all n days 150(n-8)
According to question, $ \frac { n } { 2 } [ 300 + ( n - 1 ) ( - 4 ) ] = 150 ( n - 8 )$
Hence, n = 25
View full question & answer→Question 83 Marks
A man deposited ₹ 10,000 in a bank at the rate of 5% simple interest annually. Find the amount in $15^{th}$ year since he deposited the amount and also calculate the total amount after 20 year.
AnswerTotal amount deposited = ₹ 10000, Rate of interest = 5% per annum
Interest of first year = $\frac { 10000 \times 5 \times 1 } { 100 }$ = ₹ 500
Here a = 1000, d = 500
$\therefore$ Amount in $15^{\text {th }}$ year $=\mathrm{a}_{15}=10000+(15-1) \times 500=10000+7000=₹ 17000$
Total amount after 20 years $=$ Amount in the $21^{\text {st }}$ year $=a_{21}=1000+(21-1) 500$
$=10000+10000=₹ 20000$
View full question & answer→Question 93 Marks
Let sum of $n, 2 n, 3 n$ terms of an A.P. be $S_1, S_2$ and $S_3$ respectively, show that $S_3=3\left(S_2-S_1\right)$.
AnswerGiven: ${S_1} = {n \over 2}\left[ {2a + (n - 1)d} \right]$ …..(i)${S_2} = {{2n} \over 2}\left[ {2a + (2n - 1)d} \right]$…..(ii)
And ${S_3} = {{3n} \over 2}\left[ {2a + (3n - 1)d} \right]$
Now, ${S_2} - {S_1} = {{2n} \over 2}\left[ {2a + (2n - 1)d} \right] - {n \over 2}\left[ {2a + (n - 1)d} \right]$
$\Rightarrow {S_2} - {S_1} = (n - {n \over 2})2a + \left[ {n(2n - 1) - {n \over 2}(n - 1)} \right]d$
$= na + {1 \over 2}\left[ {4{n^2} - 2n - {n^2} + n} \right]d$
$= {n \over 2}\left[ {2a + (3n - 1)d} \right] = {1 \over 3}\left\{ {{{3n} \over 2}\left[ {2a + (3n - 1)d} \right]} \right\} = {1 \over 3}{S_3}$
$\Rightarrow 3(S_2 - S_1) = S_3$
Hence proved.
View full question & answer→Question 103 Marks
A person writes a letter to four of his friends. He asks each one of them to copy the letter and mail to four different persons with instruction that they move the chain similarly. Assuming that the chain is not broken and that it costs 50 paise to mail one letter. Find the amount spent on the postage when $8^{th}$ set of letter is mailed.
AnswerTotal letters in the first set = 4, Total letters in the second set = $4^2$ = 16Total letters in the third set $= 4^3 = 64$
$\therefore$ Sequence of letters is 4, 16, 64, ………. in G.P.
Here a = 4, $r = \frac { 16 } { 4 } = 4$ and n = 8
$\therefore \quad S _ { n } = \frac { a \left( r ^ { n } - 1 \right) } { r - 1 }$
$= \frac { 4 \left( 4 ^ { 8 } - 1 \right) } { 8 - 1 }$
$= \frac { 4 } { 3 } ( 65536 - 1 )$
$= \frac { 4 } { 3 } \times 65535 = 87380$
Hence, the total number of letters mailed = 87380
The amount of postage on each letter = 50 paise
Therefore total amount spent on postage = $87380 \times 0.50$ = Rs. 43690.
View full question & answer→Question 113 Marks
Shamshad Ali buys a scooter for $₹\ 22000$. He pays $₹\ 4000$ cash and agrees to pay the balance in annual installment of $₹\ 1000$ plus $10\%$ interest on the unpaid amount. How much will the scooter cost him?
AnswerTotal cost of the scooter = ₹ 22000, Cash paid = ₹ 4000
Balance to be paid = 22000 – 4000 = ₹ 18000
Annual installment = ₹ 1000
$\therefore$ Number of installment = ${{18000} \over {1000}}$ = 18
Interest of $1^{st}$ installment = $\frac { 18000 \times 10 \times 1 } { 100 }$ = ₹ 1800
Amount of $1^{st}$ installment = 1000 + 1800 = ₹ 2800
Interest of $2^{nd}$ installment = $\frac { 17000 \times 10 \times 1 } { 100 }$ = ₹ 1700
Amount of $2^{nd}$ installment = 1000 + 1700 = ₹ 2700
Interest of $3^{rd}$ installment = $\frac { 16000 \times 10 \times 1 } { 100 }$ = ₹ 1600
Amount of $3^{rd}$ installment = 1000 + 1600 = ₹ 2600
$\therefore$ Sequence of installments is 2800, 2700, 2600, … in A.P
Here, a = 2800, d = 2700 - 2800 = -100 and n = 18
$\therefore S_n$ = $\frac n2$[2a + (n - 1) d] = $\frac {18}2$ [2 $\times$ 2800 + (18 - 1) $\times$ (-100)]
= 9 [5600 – 1700] = ₹ 35100
Therefore, the total cost of tractor is (35100 + 4000) = ₹ 39100
View full question & answer→Question 123 Marks
A farmer buys a used tractor for $₹12000$. He pays $₹ 6000$ cash and agrees to pay the balance in annual installments of $₹ 500$ plus $12\%$ interest on the unpaid amount. How much will the tractor cost him?
AnswerTotal cost of the tractor = ₹ 12000, Cash paid = ₹ 6000
Balance to be paid = 12000 – 6000 = ₹ 6000
Annual installment = ₹ 500
$\therefore$ Number of installment = ${{6000} \over {500}}$ = 12
Interest of $1^{st}$ installment = $\frac { 6000 \times 12 \times 1 } { 100 }$ = ₹ 720
Amount of $1^{st}$ installment = 500 + 720 = ₹ 1220
Interest of $2^{nd}$ installment = $\frac { 5500 \times 12 \times 1 } { 100 }$ = ₹ 660
Amount of $2^{nd}$ installment = 500 + 660 = ₹ 1160
Interest of $3^{rd}$ installment = $\frac { 5000 \times 12 \times 1 } { 100 }$ = ₹ 600
Amount of $3^{rd}$ installment = 500 + 600 = ₹ 1100
$\therefore$ Sequence of installments is 1220, 1160, 1100, ... which is in A.P
Here, a = 1220, d = 1160 - 1220 = -60 and n = 12
$\therefore S_n$ = $\frac n2$[2a + (n - 1) d]
= $\frac {12}2$[ 2 $\times$ 1220 + (12 - 1) $\times$ (-60)]
= 6 [2440 - 660] = ₹ 10680
Therefore, the total cost of tractor is (10680 + 6000) = ₹ 16680
View full question & answer→Question 133 Marks
Find the sum of the first n terms of the series: $3 + 7 + 13 + 21 + 31 + ……….$
AnswerGiven: $S_n=3+7+13+21+31+\ldots \ldots+a_{n-1}+a_n \ldots \ldots \ldots . . . .(i)$ Also
$S_n=3+7+13+21+31+\ldots \ldots .+a_{n-2}+a_{n-1}+a_n$……….(ii)
Subtracting eq. (i) from eq. (ii), $0=3+\left(4+6+8+10+\ldots . .\right.$. up to ( $n-1$ ) terms) $-a_n$
$\Rightarrow a_n=3+\frac{n-1}{2}[2 \times 4+(n-2) \times 2]$
$\Rightarrow a_n=3+\frac{n-1}{2}[8+2 n-4]$
$\Rightarrow a_n=3+(n-1)(n+2)$
$\Rightarrow a_n=3+n^2+n-2$
$\Rightarrow a_n=n^2+n+1$
$\therefore S_n=\sum_{k=1}^n a_k=\sum_{k=1}^n\left(k^2+k+1\right)$
$=\left(1^2+1+1\right)+\left(2^2+2+1\right)+\left(3^2+3+1\right)+\ldots \ldots . .+\left(n^2+n+1\right)$
$=\left(1^2+2^2+3^2+\ldots \ldots . .+n^2\right)+(1+2+3+\ldots . .+n)+n$
$= \frac { n ( n + 1 ) ( 2 n + 1 ) } { 6 } + \frac { n ( n + 1 ) } { 2 } + n$
$= n \left[ \frac { 2 n ^ { 2 } + 3 n + 1 + 3 n + 3 + 6 } { 6 } \right]$
$= n \left[ \frac { 2 n ^ { 2 } + 6 n + 10 } { 6 } \right]$
$= \frac { n } { 3 } \left( n ^ { 2 } + 3 n + 5 \right)$
View full question & answer→Question 143 Marks
Find the $20^{th}$ term of the series $2 \times 4 + 4 \times 6 + 6 \times 8 + \ldots \ldots \ldots +$ n terms.
AnswerGiven: $2 \times 4 + 4 \times 6 + 6 \times 8 + \ldots \ldots \ldots + n$terms
$\therefore a_n=\left(n^{\text {th }} \text { term of } 2,4,6, \ldots . .\right)\left(n^{\text {th }} \text { term of } 4,6,8, \ldots \ldots . .\right)$
$\Rightarrow a_n=[2+(n-1) 2][4+(n-1) 2]=2 n(2 n+2)$
$\therefore a _ { 20 } = 2 \times 20 ( 2 \times 20 + 2 ) = 40 \times 42 = 1680$
View full question & answer→Question 153 Marks
Find the sum of the series up to n terms .$6 +. 66 + .666+…$
AnswerThe given sum is not in GP but we can write it as follows: -
Sum = .6 + .66 + .666 + …to n terms
= 6(0.1) + 6(0.11) + 6(0.111) + …to n terms
taking 6 common
= 6[0.1 + 0.11 + 0.111 + …to n terms]
divide & multiply by 9,we get
= $(\frac{6}{9})$[9(0.1 + 0.11 + 0.111 + …to n terms)]
= $(\frac{6}{9})$[0.9 + 0.99 + 0.999 + …to n terms]
= $\frac{6}{9}\left[\left(\frac{9}{10}\right)+\left(\frac{99}{100}\right)+\left(\frac{999}{1000}\right)+\ldots \text { to } n \text { terms }\right]$
= $\frac{6}{9}\left[\left(1-\frac{1}{10}\right)+\left(1-\frac{1}{100}\right)+\left(1-\frac{1}{1000}\right)+\ldots \text { to } n \text { terms }\right]$
= $\frac{6}{9}[\{1+1+1+\ldots \text { to } n \text { terms }\}$ - $\left.\left\{\frac{1}{10}+\frac{1}{100}+\frac{1}{1000}+\ldots \text { to } n \text { terms }\right\}\right]$
= $\frac{6}{9}\left[n-\left\{\frac{1}{10}+\frac{1}{100}+\frac{1}{1000}+\ldots \text { to } n \text { terms }\right\}\right]$
Since $\frac{1}{10}+\frac{1}{100}+\frac{1}{1000}+\ldots \text { to } n \text { terms }$ is in GP with
first term(a) = $\frac{1}{10}$
common ratio(r) = $\frac{10^{-2}}{10^{-1}}$ = $10 ^{- 1}$ = $\frac{1}{10}$
We know that
Sum of n terms = $\frac{a\left(1-r^{n}\right)}{1-r}$ [As r < 1]
Substituting valuevalue of a & r
$\frac{1}{10}+\frac{1}{100}+\frac{1}{1000}+\ldots \text { to } n \text { terms }=\frac{a\left(1-r^{n}\right)}{1-r}$
= $\frac{\frac{1}{10}\left(1-\left(\frac{1}{10}\right)^{n}\right)}{\left(1-\frac{1}{10}\right)}$
= $\frac{\frac{1}{10}\left(1-\left(\frac{1}{10}\right)^{n}\right)}{\frac{9}{10}}$
= $\frac{1\left(1-10^{-n}\right)}{9}$Therefore, Sum = $\frac{6}{9}\left[n-\frac{1\left(1-10^{-n}\right)}{9}\right]$
View full question & answer→Question 163 Marks
Find the sum of the series up to n terms 5 + 55 + 555 + …
AnswerNow,to find sum = 5 + 55 + 555 + …. n terms.
= $\frac{5}{9}$ [9 + 99 + 999 + …. n terms]
= $\frac{5}{9}$ [(10 - 1) + (100 - 1) + (1000 - 1) + … n terms]
= $\frac{5}{9}$ [10 + 100 + 1000 ….. – (1 + 1 + … 1)]
= $\frac{5}{9}$ [10($10^n - 1$)/(10 - 1) + (1 + 1 + … n times)]
= $\frac{50}{81}$($10^n – 1$) - $\frac{5n}{9}$
View full question & answer→Question 173 Marks
If the sum of three numbers in A.P., is 24 and their product is 440. Find the numbers.
AnswerLet $(a-d), a,(a+d)$ be three numbers in A.P.
According to question, $(\mathrm{a}-\mathrm{d})+\mathrm{a}+(\mathrm{a}+\mathrm{d})=24$
$\Rightarrow 3 a=24$
$\Rightarrow a=8$
And $(a-d)(a)(a+d)=440$
$\Rightarrow\left(a^2-d^2\right) a=440$
$\Rightarrow\left(64-d^2\right) 8=440$
$\Rightarrow 64-d^2=55$
$\Rightarrow d^2=64-55$
$\Rightarrow d^2=9$
$\Rightarrow d= \pm 3$
Taking $\mathrm{d}=3$, A.P. is $(8-3), 8,(8+3)$
$\Rightarrow 5,8,11$
Taking $\mathrm{d}=-3$, A.P. is $(8+3), 8,(8-3)$
$\Rightarrow 11,8,5$
View full question & answer→Question 183 Marks
If $a$ and $b$ are the roots $x^2-3 x+p=0$ and $c, d$ are roots of $x^2-12 x+q=0$ where $a, b, c, d$ form a G.P. Prove that $(q+p):(q-p)=$ 17:15.
AnswerLet $\frac{b}{a}=\frac{c}{b}=\frac{d}{c}=\mathrm{k}$
$\therefore \frac{b}{a}=\mathrm{k}$
$\Rightarrow \mathrm{~b}=\mathrm{ak}$
And $\frac{c}{b}=k$
$\Rightarrow \mathrm{c}=\mathrm{bk}=(\mathrm{ak}) \mathrm{k}=\mathrm{ak}^2$
Also $\frac{d}{c}=\mathrm{k}$
$\Rightarrow d=c k=\left(a k^2\right) k=a k^3$
$\because a^a$ and $b^b$ are the roots $x^2-3 \mathrm{x}+p=0$
$\therefore a+b=\frac{-(-3)}{1}=3$
$\Rightarrow a+a k=3$
$\Rightarrow a(1+k)=3 \ldots \text { (i) }$
And $\mathrm{ab}=\frac{p}{1}$
$\Rightarrow a(a k)=p$
$\Rightarrow a^2 k=p \ldots \text { (ii) }$
Also $c$, $d$ are roots of $x^2-12 x+q=0$
$\therefore c+d=\frac{-(-12)}{1}=12$
$\Rightarrow a k^2+a k^3=12$
$\Rightarrow a k^2(1+k)=12 \ldots \text { (iii) }$
And $\mathrm{cd}=\frac{q}{1}$
$\Rightarrow a k^2\left(a k^3\right)=q$
$\Rightarrow a^2 k^5=q \ldots(i v)$
Dividing eq. (iii) by eq. (i), $\frac{a k^2(1+k)}{a(1+k)}=\frac{12}{3}$
$\Rightarrow k^2=4$
$\Rightarrow k= \pm 2$
Now $\frac { q + p } { q - p } = \frac { a ^ { 2 } k ^ { 5 } + a ^ { 2 } k } { a ^ { 2 } k ^ { 5 } - a ^ { 2 } k } = \frac { a ^ { 2 } k \left( k ^ { 4 } + 1 \right) } { a ^ { 2 } k \left( k ^ { 4 } - 1 \right) }$
= $\frac { ( \pm 2 ) ^ { 4 } + 1 } { ( \pm 2 ) ^ { 4 } - 1 } = \frac { 16 + 1 } { 16 - 1 } = \frac { 17 } { 15 }$
Therefore, (q + p):(q - p) = 17:15
View full question & answer→Question 193 Marks
If a, b, c, d are in G.P., prove that $(a^n + b^n), (b^n + c^n), (c^n + d^n)$ are in G.P.
AnswerGiven: a, b, c, d are in G.P.To prove: $(a^n + b^n), (b^n + c^n), (c^n + d^n)$ are in G.P.
$\Rightarrow \frac { b ^ { n } + c ^ { n } } { a ^ { n } + b ^ { n } } = \frac { c ^ { n } + d ^ { n } } { b ^ { n } + c ^ { n } }$
Let $\frac { b } { a } = \frac { c } { b } = \frac { d } { c } = k$
$\therefore \frac { b } { a } = k$
$\Rightarrow$ b = ak
And $\frac { c } { b } = k$
$\Rightarrow c = bk = (ak)k = ak^2$
Also $\frac { d } { c } = k$
$\Rightarrow d = ck = (ak^2)k = ak^3$
Now, $\frac { b ^ { n } + c ^ { n } } { a ^ { n } + b ^ { n } } = \frac { c ^ { n } + d ^ { n } } { b ^ { n } + c ^ { n } }$
$\Rightarrow \frac { ( a k ) ^ { n } + \left( a k ^ { 2 } \right) ^ { n } } { a ^ { n } + ( a k ) ^ { n } } = \frac { \left( a k ^ { 2 } \right) ^ { n } + \left( a k ^ { 3 } \right) ^ { n } } { ( a k ) ^ { n } + \left( a k ^ { 2 } \right) ^ { n } }$
$\Rightarrow \frac { a ^ { n } k ^ { n } + a ^ { n } k ^ { 2 n } } { a ^ { n } + a ^ { n } k ^ { n } } = \frac { a ^ { n } k ^ { 2 n } + a ^ { n } k ^ { 3 n } } { a ^ { n } k ^ { n } + a ^ { n } k ^ { 2 n } }$
$\Rightarrow \frac { a ^ { n } k ^ { n } \left( 1 + k ^ { n } \right) } { a ^ { n } \left( 1 + k ^ { n } \right) } = \frac { a ^ { n } k ^ { 2 n } \left( 1 + k ^ { n } \right) } { a ^ { n } k ^ { n } \left( 1 + k ^ { n } \right) }$
$\Rightarrow k^n = k^n$
Therefore, $(a^n + b^n), (b^n + c^n), (c^n + d^n)$ are in G.P.
View full question & answer→Question 203 Marks
If a$(\frac 1b + \frac 1c)$, b$(\frac 1c + \frac 1a)$, c$(\frac 1a + \frac 1b)$ are in A.P., prove that a, b, c are in A.P.
AnswerGiven: a$(\frac 1b + \frac 1c)$, b$(\frac 1c + \frac 1a)$, c$(\frac 1a + \frac 1b)$ are in A.P.
$\Rightarrow$ a$\left( \frac {b + c} {bc} \right)$, b$\left( \frac {c + a} {ca} \right) $, c$\left( \frac {a + b} {ab} \right)$ are in A.P.
$\Rightarrow \frac { a b + a c } { b c } , \frac { b c + a b } { c a } , \frac { a c + b c } { a b }$ are in A.P.
$\Rightarrow \frac {ab + ac} {bc}$ + 1, $\frac {bc + ab} {ca}$ + 1, $\frac {ac + bc} {ab}$ + 1 are in A.P. [Adding 1 to each term in the sequence]
$\Rightarrow \frac { a b + a c + b c } { b c } , \frac { b c + a b + c a } { c a } , \frac { a c + b c + a b } { a b }$ are in A.P.
$\Rightarrow \frac { 1 } { b c } , \frac { 1 } { c a } , \frac { 1 } { a b }$ are in A.P.[Dividing each fraction by ab + bc + ca]
$\Rightarrow \frac { a b c } { b c } , \frac { a b c } { c a } , \frac { a b c } { a b }$ are in A.P.[Multiplying each fraction by abc]
$\Rightarrow$ a, b, c are in A.P.
View full question & answer→Question 213 Marks
The $p^{th}, q^{th}$ and $r^{th}$ terms of an A.P. are a, b, c respectively. Show that $(q - r)a + (r - p)b + (p - q)c = 0$.
AnswerAccording to question, $a_p=a+(p-1) d=a, a_q=a+(q-1) d=b$ and $a_r=a+(r-1) d=c$
Now $(q-r) a+(r-p) b+(p-q) c=0$
Putting values of $a, b$ and $c$ we get
$(q-r)[a+(p-1) d]+(r-p)[a+(q-1) d]+(p-q)[a+(r-1) d]=0$
$\Rightarrow(q-r)[a+p d-d]+(r-p)[a+q d-d]+(p-q)[a+r d-d]=0$
$\Rightarrow a q+p q d-q d-r a-r p d+r d+a r+q r d-d r-p a-p q d+p d+p a+p r d-p d-q a-q r d+q d=0$
$\Rightarrow 0=0 \text { Proved. }$
View full question & answer→Question 223 Marks
Let S be the sum, P the product and R the sum of reciprocals of n terms in a G.P. Prove that $P^2R^n = S^n$.
AnswerLet the G.P be $a, ar, ar^2, ar^3 ..........,ar^{n - 1}$
Here $\mathrm { S } = \frac { a \left( r ^ { n } - 1 \right) } { r - 1 }$
P = $a.ar.ar^2 ......... ar^{n-1}$
$= {a^n}.{r^{1 + 2 + 3 + ....... + (n - 1)}} = {a^n}.{r^{{{n(n - 1)} \over 2}}}$
and R $= \frac { 1 } { a } + \frac { 1 } { a r } + \frac { 1 } { a r ^ { 2 } } + \ldots \ldots \frac { 1 } { a r ^ { n - 1 } }$$= {{{r^{n - 1}} + {r^{n - 2}} + {r^{n - 3}} + .......... + 1} \over {a{r^{n - 1}}}}$
$= {{1({r^n} - 1)} \over {r - 1}}.{1 \over {a{r^{n - 1}}}}$$= {{{r^n} - 1} \over {a{r^{n - 1}}(r - 1)}}$
Now ${p^2}{R^n} = {{{a^{2n}}.{r^{n(n - 1)}}{{({r^n} - 1)}^n}} \over {{a^n}{r^{n(n - 1)}}{{(r - 1)}^n}}} = {{{a^n}{{({r^n} - 1)}^n}} \over {{{(r - 1)}^n}}} = {a^n}{\left( {{{{r^n} - 1} \over {r - 1}}} \right)^n} = {S^n}$
Hence proved.
View full question & answer→Question 233 Marks
If $\frac { a + b x } { a - b x } = \frac { b + c x } { b - c x } = \frac { c + d x } { c - d x }$ (x $\neq$ 0), then show that a, b, c and d are in G.P.
AnswerTaking $\frac { a + b x } { a - b x } = \frac { b + c x } { b - c x }$
$\Rightarrow(a+b x)(b-c x)=(b+c x)(a-b x)$
$\Rightarrow a b-a c x+b^2 x-b c x^2=a b-b^2 x+a c x-b c x^2$
$\Rightarrow 2 b^2 x=2 a c x$
$\Rightarrow b^2=a c$
$\Rightarrow \frac{b}{a}=\frac{c}{b}$...(i)
Taking $\frac{b+c x}{b-c x}=\frac{c+d x}{c-d x}$
$\Rightarrow(\mathrm{b}+\mathrm{cx})(\mathrm{c}-\mathrm{dx})=(\mathrm{c}+\mathrm{dx})(\mathrm{b}-\mathrm{cx})$
$\Rightarrow 2 \mathrm{c}^2 \mathrm{x}=2 \mathrm{bdx}$
$\Rightarrow \mathrm{c}^2=\mathrm{bd}$
$\Rightarrow \frac{c}{b}=\frac{d}{c} \ldots \text { (ii) }$
From eq. (i) and (ii), $\frac{b}{a}=\frac{c}{b}=\frac{d}{c}$
View full question & answer→Question 243 Marks
The sum of the first four terms of an A.P. is $56$. The sum of the last four terms is $112$. If its first term is $11$, then find the number of terms.
Answer$\text { Given: } a=11 \text { and } S_4=56$
$\Rightarrow S_4=\frac{4}{2}[2 \times 11+(4-1) d]$
$\Rightarrow 2(22+3 d)=56$
$\Rightarrow 22+3 d=28$
$\Rightarrow 3 d=6$
$\Rightarrow d=2$
$\text { Also, } I+(I-d)+(I-2 d)+(I-3 d)=112$
$\Rightarrow 4 \mid-6 d=112$
$\Rightarrow 4 I=112+6 d$
$\Rightarrow 4 l=112+6 \times 2$
$\Rightarrow 4 \mid=112+12$
$\Rightarrow 4 I=124$
$\Rightarrow I=31$
$\therefore a_n=a+(n-1) d$
$\Rightarrow 31=11+(n-1) \times 2$
$\Rightarrow 2(n-1)=20$
$\Rightarrow n-1=10$
$\Rightarrow n=11$
View full question & answer→Question 253 Marks
A G.P. consists of an even number of terms. If the sum of all the terms is 5 times the sum of terms occupying odd places, then find its common ratio.
AnswerLet the number of terms be 2n then we have the number of odd terms is n
Let the G.P be $a, ar, ar^2, .... ar^{2n-1}$
Then the odd terms $a, ar^2, ar^4, ar^6$, .... form a G.P
$\therefore S_{2n}$ = $\frac {a \left(r ^ {2n} - 1\right)} {r - 1}$ and $S_n$ = a$\left[ {{{{{({r^2})}^n} - 1} \over {{r^2} - 1}}} \right]$
According to question, $S_{2n} = 5S_n$
$\Rightarrow$ $a\left[ {{{{r^{2n}} - 1} \over {r - 1}}} \right] = 5a\left[ {{{{{({r^2})}^n} - 1} \over {{r^2} - 1}}} \right]$
$\Rightarrow {1 \over {r - 1}} = {5 \over {{r^2} - 1}}$
$\Rightarrow$ r + 1 = 5
$\Rightarrow$ r = 4
View full question & answer→Question 263 Marks
The sum of three numbers in G.P. is $56$. If we subtract $1, 7, 21$ from these numbers in that order, we obtain an arithmetic progression. Find the numbers.
AnswerLet $a, a r, a r^2$ be three numbers in G.P., therefore, $a+a r+a r^2=56 \Rightarrow a\left(1+r+r^2\right)=56$ . . . . .(i)
According to question, $a-1$, ar $-7, a r^2-21$ are in A.P.
$\therefore(a r-7)-(a-1)=\left(a r^2-21\right)-(a r-7)$
$\Rightarrow a r-7-a+1=a r^2-21-a r+7$
$\Rightarrow a r-a-6=a r^2-a r-14$
$\Rightarrow a r^2-2 a r+a=8$
$\Rightarrow a\left(r^2-2 r+1\right)=8 \ldots . . . . .(i i)$
Dividing eq. (i) by eq. (ii), $\frac{a\left(1+r+r^2\right)}{a\left(r^2-2 r+1\right)}=\frac{56}{8}$
$\Rightarrow 1+r+r^2=7 r^2-14 r+7$
$\Rightarrow 6 r^2-15 r+6=0$
$\Rightarrow 2 r^2-5 r+2=0$
$\Rightarrow r = \frac { - ( - 5 ) \pm \sqrt { ( - 5 ) ^ { 2 } - 4 \times 2 \times 2 } } { 2 \times 2 }$
$= \frac { 5 \pm \sqrt { 25 - 16 } } { 4 } = \frac { 5 \pm \sqrt { 9 } } { 4 } = \frac { 5 \pm 3 } { 4 }$=
$\Rightarrow r = \frac { 5 + 3 } { 4 } = \frac { 8 } { 4 } = 2$or $r = \frac { 5 - 3 } { 4 } = \frac { 2 } { 4 } = \frac { 1 } { 2 }$
Putting r = 2 in eq. (i), a$(1 + 2 + 2^2) = 56$
$\Rightarrow a = \frac { 56 } { 7 } = 8$
Then the required numbers are 8, 16, 32.
Putting $r = \frac { 1 } { 2 }$ in eq. (i), $a \left( 1 + \frac { 1 } { 2 } + \frac { 1 } { 4 } \right) = 56$
$\Rightarrow a \times \frac { 7 } { 4 } = 56$
$\Rightarrow$ a = 32
Then the required numbers are $32, 16, 8$.
View full question & answer→Question 273 Marks
Show that the sum of $(m+n)^{\text {th }}$ and $(m-n)^{\text {th }}$ terms of an A.P. is equal to twice the $m^{\text {th }}$ terms.
AnswerHere, $a_{m+n}=a+(m+n-1) d$ …..(i) and $a_{m-n}=a+(m-n-1) d$ …..(ii)
To prove: $a_{m+n}+a_{m-n}=2 a_m$
Adding eq. (i) and (ii), we get
$a_{m+n}+a_{m-n}=a+(m+n-1) d+a+(m-n-1) d$
$\Rightarrow a_{m+n}+a_{m-n}=a+m d+n d-d+a+m d-n d-d$
$\Rightarrow a_{m+n}+a_{m-n}=2 a+2 m d+-2 d$
$\Rightarrow a_{m+n}+a_{m-n}=2(a+m d-d)$
$\Rightarrow a_{m+n}+a_{m-n}=2[a+(m-1) d]$
$\Rightarrow a_{m+n}+a_{m-n}=2 a_m$
View full question & answer→Question 283 Marks
Find the sum to n terms in each of the series in whose $n^{th}$ terms is given by $n(n + 1)(n + 4)$
AnswerGiven: $an = n(n + 1) (n + 4) = n^3 + 5n^2 + 4n \therefore {S_n} = \sum\limits_{k = 1}^n {{a_k}} = \sum\limits_{k = 1}^n {{k^3} + 5{k^2} - 4k} $
$=\left(1^3+5.1^2+4.1\right)+\left(2^3+5.2^2+4.2\right)+\ldots \ldots .\left(n^3+5 . n^2+4 . n\right)$
$=\left(1^3+2^3+\ldots . .+n^3\right)+5\left(1^2+2^2+\ldots .+n^2\right)+4(1+2+3+\ldots \ldots . .+n)$
$= {\left[ {{{n(n + 1)} \over 2}} \right]^2} + {{5n(n + 1)(2n + 1)} \over 6} + {{4n(n + 1)} \over 2}$
$= \frac { n ^ { 2 } ( n + 1 ) ^ { 2 } } { 4 } + \frac { 5 n ( n + 1 ) ( 2 n + 1 ) } { 6 } + 2 n ( n + 1 )$
$= n ( n + 1 ) \left[ \frac { n ( n + 1 ) } { 4 } + \frac { 5 ( 2 n + 1 ) } { 6 } + 2 \right]$
$= n(n + 1)\left[ {{{3{n^2} + 3n + 20n + 10 + 24} \over {12}}} \right]$
$= \frac { n ( n + 1 ) \left( 3 n ^ { 2 } + 23 n + 34 \right) } { 12 }$
View full question & answer→Question 293 Marks
Find the sum to n terms of the series $1 ^ { 2 } + \left( 1 ^ { 2 } + 2 ^ { 2 } \right) + \left( 1 ^ { 2 } + 2 ^ { 2 } + 3 ^ { 2 } \right) + \ldots \ldots$
AnswerGiven: $1^2 + (1^2 + 2^2) +(1^2 + 2^2 + 3^2)+$....... to $n$ terms$\therefore a^n = (1^2 + 2^2 + 3^2 + ....... n^2)$
= $\frac { n ( n + 1 ) ( 2 n + 1 ) } { 6 }$
= $\frac { 1 } { 6 } \left( 2 n ^ { 3 } + 3 n ^ { 2 } + n \right)$
${\therefore} \ {{\text{S}}_n} = \sum\limits_{k = 1}^n {{a_k}} = \sum\limits_{k = 1}^n {\frac{1}{6}} \left( {2{k^3} + 3{k^2} + k} \right)$
$=\frac { 1 } { 6 } \left( 2.1 ^ { 3 } + 3.1 ^ { 2 } + 1 \right) + \frac { 1 } { 6 } \left( 2.2 ^ { 3 } + 3.2 ^ { 2 } + 2 \right) + \ldots \ldots + \frac { 1 } { 6 } \left( 2 . n ^ { 3 } + 3 n ^ { 2 } + n \right)$
$=\frac { 1 } { 6 } \left[ 2 \left( 1 ^ { 3 } + 2 ^ { 3 } + \ldots \ldots + n ^ { 3 } \right) + 3 \left( 1 ^ { 2 } + 2 ^ { 2 } + \ldots \ldots + n ^ { 2 } \right) + ( 1 + 2 + 3 + \ldots \ldots + n ) \right]$
$= \frac { 1 } { 6 } \left[ 2 \left\{ \frac { n ( n + 1 ) } { 2 } \right\} ^ { 2 } + \frac { 3 n ( n + 1 ) ( 2 n + 1 ) } { 6 } + \frac { n ( n + 1 ) } { 2 } \right]$
$= \frac { n ( n + 1 ) } { 12 } \left[ \frac { n ( n + 1 ) + 2 n + 1 + 1 } { 1 } \right]$
$= \frac { n ( n + 1 ) } { 12 } \left( n ^ { 2 } + 3 n + 2 \right)$
$= \frac { n ( n + 1 ) ( n + 1 ) ( n + 2 ) } { 12 }$
$= \frac { n ( n + 1 ) ^ { 2 } ( n + 2 ) } { 12 }$
View full question & answer→Question 303 Marks
Find the sum to n terms of the series $1 \times 2 \times 3 + 2 \times 3 \times 4 + 3 \times 4 \times 5 + \ldots$
AnswerGiven: $1 \times 2 \times 3 + 2 \times 3 \times 4 + 3 \times 4 \times 5 + \ldots$ to n terms$\therefore a_n = [n^{th}$ term of $1, 2, 3, ………..][n^{th}$ term of $2, 3, 4, 5,$ ………][$n^{th}$ term of $3, 4, 5, ………]$
$= [ 1 + ( n - 1 ) \times 1 ] [ 2 + ( n - 1 ) \times 1 ] [ 3 + ( n - 1 ) \times 1 ]$
$= n(n + 1) (n + 2) = n^3 + 3n^2 + 2n$
${\therefore } \ {{\text{S}}_n} = \sum\limits_{k = 1}^n {{a_k}} = \sum\limits_{k = 1}^n {\left( {{k^3} + 3{k^2} + 2k} \right)} $
$=\left[1^3+3.1^2+2.1\right]+\left[2^3+3.2^2+2.2\right]+\left[3^3+3.3^2+2.3\right]+\ldots \ldots . .\left[n^3+3 n^2+2 n\right]$ $=\left[1^3+2^3+3^3+\ldots . . . . . n^3\right]+3\left[1^2+2^2+3^2+\ldots . . . n^2\right]+2[1+2+3+\ldots . . . n n]$
$= {\left[ {{{n(n + 1)} \over 2}} \right]^2} + {{3n(n + 1)(2n + 1)} \over 6} + {{2n(n + 1)} \over 2}$
$= \frac { n ^ { 2 } ( n + 1 ) ^ { 2 } } { 4 } + \frac { n ( n + 1 ) ( 2 n + 1 ) } { 2 } + n ( n + 1 )$
$= { n ( n + 1 ) } \left[ \frac { n ( n + 1 ) } { 4 } + \frac { 2 n + 1 } { 2 } + 1 \right]$
= $n ( n + 1 ) \left[ \frac { n ^ { 2 } + n + 4 n + 2 + 4 } { 4 } \right]$
$= {{n(n + 1)({n^2} + 5n + 6)} \over 4}$
$= \frac { 1 } { 4 } n ( n + 1 ) ( n + 2 ) ( n + 3 )$
View full question & answer→Question 313 Marks
Find the sum to n terms in each of the series in whose $n^{th}$ terms is given by $(2n - 1)^2$
AnswerGiven: $a_n = (2n - 1)^2 = 4n^2 - 4n + 1 \therefore {S_n} = \sum\limits_{k = 1}^n {{a_k}} \sum\limits_{k = 1}^n {4{k^2} - 4k + 1} $
$= (4.1^2 - 4.1 + 1) + (4.2^2 - 4.2 + 1) + ......... (4.n^2 - 4.n + 1)$
$= 4(1^2 + 2^2 + ........ + n^2) - 4(1 + 2 + 3 + ...... + n) + (1 + 1 + ....... n terms)$
$= {{4n(n + 1)(2n + 1)} \over 6} - {{4n(n + 1)} \over 2} + n$
$= n\left( {{{2(n + 1)(2n + 1)} \over 3} - 2(n + 1) + 1} \right)$
$= n \left( \frac { 4 n ^ { 2 } + 6 n + 2 - 6 n - 6 + 3 } { 3 } \right)$
$= n \left( \frac { 4 n ^ { 2 } - 1 } { 3 } \right)$
$= \frac { n ( 2 n + 1 ) ( 2 n - 1 ) } { 3 }$
View full question & answer→Question 323 Marks
Find the sum to n terms of the series $1 \times 2+2 \times 3+3 \times 4+4 \times 5+. .$
AnswerGiven,$an = n(n + 1) = n^2 + n$
Sn = $\sum_{n=1}^{n} a_{n}$
= $\sum_{n=1}^{n} n^{2}+n$
= $\sum_{n=1}^{n} n^{2}+\sum_{n=1}^{n} n$
= $\frac{n(n+1)(2 n+1)}{6}+\frac{n(n+1)}{2}$
= $\frac{\mathrm{n}(\mathrm{n}+1)}{2}\left(\frac{(2 \mathrm{n}+1)}{3}+1\right)$
= $\frac{n(n+1)}{2}\left(\frac{2 n+1+3}{3}\right)$
= $\frac{n(n+1)}{2}\left(\frac{2 n+4}{3}\right)$
= $\frac{n(n+1)}{2 \times 3}[2(n+2)]$
= $\frac{2 n(n+1)(n+2)}{6}$
Therefore, the required sum is $\frac{2 n(n+1)(n+2)}{6}$
View full question & answer→Question 333 Marks
Find the sum of n terms of the geometric progression $1, -a, a^2, -a^3 ... n$ terms (if a $\ne$ -1).
AnswerHere, a = 1 and r = $\frac {-a} {1}$ = -a
$\therefore ^ { \mathrm { S } _ { n } = \frac { a \left( 1 - r ^ { n } \right) } { 1 - r } }$ when r < 1
$\Rightarrow S_n$ = $\frac { 1 \left[ 1 - (-a) ^ { n } \right] } { 1 - (-a) }$
$\Rightarrow S_n$ = $\frac { 1 } { 1 + a }$[$1 - (-a)^n$]
View full question & answer→Question 343 Marks
Find the sum to indicated number of terms of the geometric progression $ \sqrt { 7 } , \sqrt { 21 } , 3 \sqrt { 7 }$, ... n terms.
AnswerHere,a = $ \sqrt 7$ and r =$ \frac { \sqrt { 21 } } { \sqrt { 7 } } = \sqrt { 3 }$
$ \therefore S_ { n } = \frac { a \left( r ^ { n } - 1 \right) } { r - 1 }$ when r > 1
$ \Rightarrow \mathrm { S } _ { n } = \frac { \sqrt { 7 } \left[ ( \sqrt { 3 } ) ^ { n } - 1 \right] } { \sqrt { 3 } - 1 }$
$ S _ { n } = \frac { \sqrt { 7 } } { \sqrt { 3 } - 1 } \times \frac { \sqrt { 3 } + 1 } { \sqrt { 3 } + 1 } \left[ ( 3 ) ^ { \frac { n } { 2 } } - 1 \right]$
$ \Rightarrow \mathrm { S } _ { n } = \frac { \sqrt { 7 } ( \sqrt { 3 } + 1 ) } { 2 } \left[ ( 3 ) ^ { \frac { n } { 2 } } - 1 \right]$
View full question & answer→Question 353 Marks
Find the sum to indicated number of terms of the geometric progression 0.15, 0.015, 0.0015, ... 20 terms.
AnswerHere,a = 0.15 and r = $\frac { 0.015 } { 0.15 } = \frac { 1 } { 10 }$
$\mathrm { S } _ { n } = \frac { a \left( 1 - r ^ { n } \right) } { 1 - r }$ when r < 1
$\Rightarrow S _ { 20 } = \frac { 0.15 \left[ 1 - \left( \frac { 1 } { 10 } \right) ^ { 20 } \right] } { 1 - \frac { 1 } { 10 } }$
$\Rightarrow \mathrm { S } _ { 20 } = \frac { 15 } { 100 } \times \frac { 10 } { 9 } \left[ 1 - ( 0.1 ) ^ { 20 } \right]$
$\Rightarrow \mathrm { S } _ { 20 } = \frac { 1 } { 6 } \left[ 1 - ( 0.1 ) ^ { 20 } \right]$
View full question & answer→Question 363 Marks
Which term of the sequence $\sqrt { 3 } , 3,3 \sqrt { 3 }$ , ..... is 729?
AnswerHere a = $\sqrt3$ , r = $\frac { 3 } { \sqrt { 3 } } = \sqrt { 3 }$ and $a_n = 729$
$\therefore a_n = ar^{n-1}$
$\Rightarrow 729 = \sqrt { 3 } \times ( \sqrt { 3 } ) ^ { n - 1 }$
$\Rightarrow ( \sqrt { 3 } ) ^ { 12 } = ( \sqrt { 3 } ) ^ { n }$
$\Rightarrow$n = 12
Therefore, $12^{th}$ term of the given G.P. is 729.
View full question & answer→Question 373 Marks
If A.M. and G.M. of roots of a quadratic equation are 8 and 5 respectively then obtain the quadratic equation.
AnswerLet a and b be the roots of required quadratic equation.
Then A.M. $=\frac{a+b}{2}=8$
$\Rightarrow$
$a+b=16$
$\text { And G.M. }=\sqrt{a b}=5$
$\Rightarrow a b=25$
Now, Quadratic equation $x^2-$ (Sum of roots) $x+$ (Product of roots) $=0$
$\Rightarrow x^2-(a+b) x+a b=0$
$\Rightarrow x^2-16 x+25=0$
Therefore, required equation is $x^2-16 x+25=0$
View full question & answer→Question 383 Marks
What will ₹ 500 amount to 10 years after its deposit in a bank which pays annual interest rate of 10% compounded annually?
AnswerOriginal amount = ₹ 500, Rate of interest = 10% compounded annually
$\therefore$ Interest of one year = $\frac { 500 \times 10 \times 1 } { 100 }$ = ₹ 50
And Amount after one year = 500 + 50 = ₹ 550
Here a = 500 and r = $\frac { 550 } { 500 }$ = 1.1
Therefore, amount after 10 years = Amount in the $11^{th}$ year
$= 5.5 \times (1.1)^{11 - 1} = ₹ 500(1.1)^{10}$
View full question & answer→Question 393 Marks
The number of bacteria in a certain culture doubles every hour. If there were 30 bacteria present in the culture originally, how many bacteria will be present at the end of $2^{nd}$ hour, $4^{th}$ hour and $n^{th}$ hour?
AnswerBacteria present in the culture originally = 30Since the bacteria doubles itself after each hour, then the sequence of bacteria after each hour is a G.P.
Here a = 30 and r = 2
$\therefore$ Bacteria at the end of $2^{nd}$ hour = $30 \times 2 ^ { 3 - 1 } = 30 \times 2 ^ { 2 } = 120$
And Bacteria at the end of $4^{th}$ hour = $30 \times 2 ^ { 5 - 1 } = 30 \times 2 ^ { 4 } = 480$
And Bacteria at the end of nth hour =${a_{n + 1}} = 30({2^{(n + 1) - 1}}) = 30({2^n})$
View full question & answer→Question 403 Marks
If A and G be A.M. and G.M. respectively between two positive numbers, prove that the numbers are $\mathrm { A } \pm \sqrt { ( \mathrm { A } + \mathrm { G } ) ( \mathrm { A } - \mathrm { G } ) }$.
AnswerLet the two positive numbers be a and b Therefore A = $\frac { a + b } { 2 }$ and G = $\sqrt { a b }$
Now, $\mathrm { A } \pm \sqrt { ( \mathrm { A } + \mathrm { G } ) ( \mathrm { A } - \mathrm { G } ) } = \mathrm { A } \pm \sqrt { \mathrm { A } ^ { 2 } - \mathrm { G } ^ { 2 } }$
= $\frac { a + b } { 2 } \pm \sqrt { \left( \frac { a + b } { 2 } \right) ^ { 2 } - ( \sqrt { a b } ) ^ { 2 } }$
= $\frac { a + b } { 2 } \pm \sqrt { \frac { a ^ { 2 } + b ^ { 2 } + 2 a b } { 4 } - a b }$
= $\frac { a + b } { 2 } \pm \sqrt { \frac { a ^ { 2 } + b ^ { 2 } + 2 a b - 4 a b } { 4 } }$
= $\frac { a + b } { 2 } \pm \sqrt { \frac { ( a - b ) ^ { 2 } } { 4 } } = \frac { a + b } { 2 } \pm \frac { a - b } { 2 }$
= $\frac { a + b } { 2 } + \frac { a - b } { 2 }$ and $\frac { a + b } { 2 } - \frac { a - b } { 2 }$
= $\frac { a + b + a - b } { 2 }$ and $\frac { a + b - a + b } { 2 }$
= $\frac { 2 a } { 2 } = a$ and $\frac { 2 b } { 2 } = b$
View full question & answer→Question 413 Marks
The sum of two numbers is 6 times their geometric mean, show that numbers are in the ratio $( 3 + 2 \sqrt { 2 } ):( 3 - 2 \sqrt { 2 } )$.
AnswerLet the numbers be a and b
Given: a + b = 6$\sqrt { a b } \Rightarrow \frac { a + b } { 2 \sqrt { a b } } = \frac { 3 } { 1 }$
Applying componendo and dividendo, we get
$\frac { a + b + 2 \sqrt { a b } } { a + b - 2 \sqrt { a b } } = \frac { 3 + 1 } { 3 - 1 }$
$\Rightarrow \frac { ( \sqrt { a } + \sqrt { b } ) ^ { 2 } } { ( \sqrt { a } - \sqrt { b } ) ^ { 2 } } = \frac { 4 } { 2 }$
$\Rightarrow \frac { \sqrt { a } + \sqrt { b } } { \sqrt { a } - \sqrt { b } } = \frac { \sqrt { 2 } } { 1 }$
Again applying componendo and dividendo, we get
${{\sqrt a + \sqrt b + \sqrt a - \sqrt b } \over {\sqrt a + \sqrt b - \sqrt a + \sqrt b }} = {{\sqrt 2 + 1} \over {\sqrt 2 - 1}}$
$\Rightarrow \frac { \sqrt { a } } { \sqrt { b } } = \frac { \sqrt { 2 } + 1 } { \sqrt { 2 } - 1 }$
Squaring both sides, $\frac { a } { b } = \frac { 2 + 1 + 2 \sqrt { 2 } } { 2 + 1 - 2 \sqrt { 2 } }$
$\Rightarrow \frac { a } { b } = \frac { 3 + 2 \sqrt { 2 } } { 3 - 2 \sqrt { 2 } }$
Therefore, the numbers are in the ratio $( 3 + 2 \sqrt { 2 } ) : ( 3 - 2 \sqrt { 2 } )$
View full question & answer→Question 423 Marks
Find the value of n so that $\frac{a^{n+1}+b^{n+1}}{a^{n}+b^{n}}$ may be the geometric mean between a and b.
Answer$\frac{a^{n+1}+b^{n+1}}{a^{n}+b^{n}}=\sqrt{a b}$
$\frac{a^{n+1}+b^{n+1}}{a^{n}+b^{n}}=\frac{a^{\frac{1}{2}} b^{\frac{1}{2}}}{1}$
$a^{n+1}+b^{n+1}=a^{\frac{1}{2}} b^{\frac{1}{2}}\left(a^{n}+b^{n}\right)$ $a^{n+1}+b^{n+1}=a^{n+\frac{1}{2}} b^{\frac{1}{2}}+a^{\frac{1}{2}} b^{n+\frac{1}{2}}$
$a^{n+1}-a^{n+\frac{1}{2}} b^{\frac{1}{2}}=a^{\frac{1}{2}} b^{n+\frac{1}{2}}-b^{n+1}$
${a^{n + \frac{1}{2}}}\left( {{a^{\frac{1}{2}}} - {b^{\frac{1}{2}}}} \right) = {b^{n + \frac{1}{2}}}\left( {{a^{\frac{1}{2}}} - {b^{\frac{1}{2}}}} \right)$
$\left(\frac{a}{b}\right)^{n+\frac{1}{2}}=1$
$\left(\frac{a}{b}\right)^{n+\frac{1}{2}}=\left(\frac{a}{b}\right)^{0}$
$n+\frac{1}{2}=0$
$n=\frac{-1}{2}$
View full question & answer→Question 433 Marks
Insert two numbers between 3 and 81, so that resulting sequence is GP.
AnswerLet the two numbers be $a$ and $b$, then $3, a, b, 81$ are in GP.
$\because n^{\text {th }} \text { term, } T_n=A r^{n-1}$
$\therefore T_4=81=3 r^{4-1} \Rightarrow r^3=\frac{81}{3}$
$\Rightarrow r^3=27 \Rightarrow r^3=3^3$
On comparing the base of power 3 from both sides, we get
$r=3$
$\therefore a=A r=3 \times 3=9$
$\text { and } b=A r^2=3 \times 3^2=27$
View full question & answer→Question 443 Marks
If $a, b, c$ and $d$ are in G.P show that $\left(a^2+b^2+c^2\right)\left(b^2+c^2+d^2\right)=(a b+b c+c d)^2$.
Answera, b, c, d are in G. P
$b=a r$
$\mathrm{c}=\mathrm{ar}^2$
$d=a r^3$
$\text { L. H. S }=\left(a^2+b^2+c^2\right)\left(b^2+c^2+d^2\right)$
$=\left(a^2+a^2 r^2+a^2 r^4\right)\left(a^2 r^2+a^2 r^4+a^2 r^6\right)$
$=a^4 r^2\left(1+r^2+r^4\right)^2$
R. H.S. $=(a b+b c+c d)^2$
$=\left(a^2 r+a^2 r^3+a^2 r^5\right)^2$
$=a^4 r^2\left(1+r^2+r^4\right)^2$
H.p
View full question & answer→Question 453 Marks
Show that the ratio of the sum of first n terms of a G.P. to the sum of terms from $(n + 1)^{th}$ to $(2n)^{th}$ term is $\frac { 1 } { r ^ { n } }$.
AnswerLet a be the first term and r be the common ratio of given G.P.
Then $\frac { \text { Sum of first } n \text { terms } } { \text { Sum of terms from } ( n + 1 ) ^ { \text { th } } \text { to } ( 2 n ) ^ { \frac { k } { n ^ { 2 } } } }$
= ${{a + ar + a{r^2} + .... + a{r^{n - 1}}} \over {a{r^n} + a{r^{n + 1}} + ... + a{r^{2n - 1}}}}$
= $\frac { a + a r + a r ^ { 2 } + \ldots + a r ^ { n - 1 } } { r ^ { n } \left[ a + a r + a r ^ { 2 } + \ldots + a r ^ { n - 1 } \right] } = \frac { 1 } { r ^ { n } }$
View full question & answer→Question 463 Marks
If the first and the $n^{\text {th }}$ term of a G.P. are $a$ and $b$ respectively and if $P$ is the product of $n$ terms, prove that $P^2=(a b)^n$.
AnswerLet $r$ be the common ratio of the given G.PHere, first term of G.P. is a and $a_n=b$
$\Rightarrow \mathrm{ar}^{\mathrm{n}-1}=\mathrm{b}$ ……….(i)
Given: $P=a \cdot a r . a r^2 \cdot a r^3 . . . . . . a r^{n-1}$
$\Rightarrow \mathrm{P}=\mathrm{a}^n \cdot r^{1+2+3+\ldots . . .+n-1}$
$\Rightarrow p=a^n r^{\frac{n(n-1)}{2}}$
$\Rightarrow p^2=a^{2 n} r^{n(n-1)}=\left[a^{n-1} r^n[\text { Squaring both sides }]\right.$
$\Rightarrow \mathrm{P}^2=(\mathrm{ab})^n[\text { From eq. (i) }]$
Hence proved
View full question & answer→Question 473 Marks
If the pth, $q$ th and $r$ th terms of a G.P. are $a, b$ and $c$ respectively. Prove that $a^{q-r} b^{r-p} c^{p-q}=1$.
AnswerLet $A$ be the first term and $R$ be the common ratio of given G.P. $\therefore a_p=a$
$\Rightarrow A R^{p-1}=a$……….(i)
$a_q=b$
$\Rightarrow A R^{q-1}=b$……….(ii)
$a_r=c$
$\Rightarrow A R^{r-1}=c$……….(iii)
Now, L.H.S. $=a^{q-r} b^{r-p} c^{p-q}=\left(A^{p-1}\right)^{q-r} \cdot\left(A R^{q-1}\right)^{r-p} \cdot\left(A^{r-1}\right)^{p-q}$
$=A^{q-r} R^{(p-1)(q-r)} \cdot A^{r-p} R^{(q-1)(r-p)} \cdot A^{p-q} R^{(r-1)(p-q)}$
$=A^{q-r+r-p+p-q} R^{p q-p r-q+r+q r-p q-r+p+p r-q r-p+q}$
$=A^0 R^0=1 \times 1=1=\text { R.H.S. }$
View full question & answer→Question 483 Marks
Find four numbers forming a geometric progression in which the third term is greater than the first term by $9$ and the second term is greater than by $4^{th}$ by $18$.
AnswerLet the four numbers in G.P. be a, $a r, a r^2, a r^3$
$\therefore a r^2=a+9 \text { and } a r=a r^3+18$
Now, $a r^2-a=9$
$\Rightarrow a\left(r^2-1\right)=9 .$...(i)
And $a r-a r^3=18$
$\Rightarrow \operatorname{ar}\left(1-r^2\right)=18+$
$\Rightarrow-\operatorname{ar}\left(r^2-1\right)=18$...(ii)
Dividing eq. (ii) by eq. (i), we have
$\frac{-a r\left(r^2-1\right)}{a\left(r^2-1\right)}=\frac{18}{9}$
$\Rightarrow \mathrm{r}=-2$
Putting value of $r$ in eq. (i), we get
$a(4-1)=9$
$\Rightarrow a=3$
$\therefore a r=3 \times(-2)=-6$
$a r^2=3 \times(-2)^2=12 a r^2\{3\}$
$=3 \times(-2)^3=-24$
Therefore, the required numbers are $3,-6,12,-24$
View full question & answer→Question 493 Marks
Show that the products of the corresponding terms of the sequences $\mathrm{a}, \mathrm{ar}, \mathrm{ar}^2$,............ $a r^{n-1}$ and $A, A R, A R^2$............. $A R^{n-1}$ form a G.P. and find the common ratio.
AnswerMultiplying the corresponding terms of the given sequences, we have$( a \times \mathrm { A } ) , ( a r \times \mathrm { AR } ) _ { : } \left( a r ^ { 2 } \times \mathrm { AR } ^ { 2 } \right),$........,$\left( a r ^ { n - 1 } \times \mathrm { AR } ^ { n - 1 } \right)$
$\Rightarrow(A),\left(a A r^R\right),\left(a A r^2 R^2\right), \ldots \ldots,\left(a A r^{n-1} R^{n-1}\right)$ are in G.P.
Now ${{{a_2}} \over {{a_1}}} = {{aArR} \over {aA}} = rR$ and ${{{a_3}} \over {{a_2}}} = {{aA{r^2}{R^2}} \over {aArR}} = rR$
Since the ratio of the two succeeding terms are same, the resulting sequence is also in G.P
and common ratio = $\frac { a \mathrm { A } r \mathrm { R } } { a \mathrm { A } } = r \mathrm { R }$
View full question & answer→Question 503 Marks
Find the sum of the product of the corresponding terms of the sequences $2,4,8,16,32$ and $128,32,8,2, \frac{1}{2}$.
AnswerMultiplying the corresponding terms of the given sequences $2,4,8,16,32$ and $128,32,8,2, \frac{1}{2}$
$(2 \times 128),(4 \times 32),(8 \times 8),(16 \times 2),\left(32 \times \frac{1}{2}\right)$
$\Rightarrow 256,128,64,32,16 \text { are in G.P. }$
Here $\mathrm{a}=256, \mathrm{r}=\frac{128}{256}=\frac{1}{2}$ and $\mathrm{n}=5$
$\therefore \mathrm{S}_n=\frac{a\left(1-r^n\right)}{1-r} \text { when } \mathrm{r}<1$
$\mathrm{~S}_5=\frac{256\left[1-\left(\frac{1}{2}\right)^5\right]}{1-\frac{1}{2}}=256 \times 2\left(1-\frac{1}{32}\right)$
$\Rightarrow \mathrm{S}_5=256 \times 2 \times \frac{31}{32}=496$
View full question & answer→Question 513 Marks
Find the sum to n terms of the sequences $8, 88, 888, 8888$, ……
AnswerHere $Sn = 8 + 88 + 888 + 8888 +$ ....... up to $n$ terms$\Rightarrow Sn = 8(1 +11 + 111 + 1111 +$ ...... up to $n$ terms)
$\Rightarrow S _ { n } = \frac { 8 } { 9 }$(9 + 99 + 999 + 9999 + ....... up to n terms) 
$\Rightarrow S _ { n } = \frac { 8 } { 9 } \left[ ( 10 - 1 ) + \left( 10 ^ { 2 } - 1 \right) + \left( 10 ^ { 3 } - 1 \right) + \ldots . \text { up to } n \text { terms } \right]$
$\Rightarrow \mathrm { S } _ { n } = \frac { 8 } { 9 }[(10 + 10^2 + 10^3 +$ ..... up to $n$ terms) - (1 + 1 + 1 + ...... up to n terms)]
$\Rightarrow S _ { n } = \frac { 8 } { 9 } \left[ \frac { 10 \times \left( 10 ^ { n } - 1 \right) } { 10 - 1 } - n \right]$
$= \frac { 8 } { 9 } \left[ \frac { 10 } { 9 } \left( 10 ^ { n } - 1 \right) - n \right]$
$= \frac { 80 } { 81 } \left( 10 ^ { n } - 1 \right) - \frac { 8 } { 9 } n$ View full question & answer→Question 523 Marks
If the $4^{\text {th }}, 10^{\text {th }}$ and $16^{\text {th }}$ terms of a G.P. are $x, y$ and $z$ respectively. Prove that $x, y, z$ are in G.P.
AnswerLet $a$ be the first term and $r$ be the common ratio of given G.P.
$\therefore a_4=x$
$\Rightarrow a r^3=x \ldots \text { (i) }$
$a_{10}=y$
$\Rightarrow a r^9=y \ldots \text { (ii) }$
$a_{16}=z$
$\Rightarrow a r^{15}=z \ldots \text { (iii) }$
From eq. (ii), $a r^9=y$
$\Rightarrow\left(a r^9\right)^2=y^2$
$\Rightarrow y^2=\left(a r^3\right)\left(a r^{15}\right)$
$\Rightarrow y^2=x z[\text { From eq. (i) and (iii)] }$
$\therefore x, y, z \text { are in G.P. }$
View full question & answer→Question 533 Marks
Find a G.P. for which sum of the first two terms is -4 and the fifth term is 4 times the third term.
AnswerLet $a$ be the first term and $r$ be the common ratio of given G.P.Given: $a+a r=-4$
$\Rightarrow a(1+r)=-4$……..(i)
$\text { And } a_5=4 a_3$
$\Rightarrow a r^4=4 r^2$
$\Rightarrow r^2=4$
$\Rightarrow r= \pm 2$$\qquad$
Putting $\mathrm{r}=2 r=2$ in eq. (i), we get $\mathrm{a}(1+2)=-4$
$\Rightarrow a=\frac{-4}{3}$
Therefore, required G.P. is $\frac{-4}{3}, \frac{-8}{3}, \frac{-16}{3}, \ldots$
Putting $r=-2$ in eq. (i), we get $a(1-2)=-4$
$\Rightarrow a=4$
Therefore, required G.P. is $4,-8,16,-32, \ldots . . .$.
View full question & answer→Question 543 Marks
Given a G.P. with a = 729 and $7^{th}$ term 64, determine $S_7$ .
AnswerGiven: a = 729 and $a_7$ = 64$\Rightarrow a r ^ { 6 } = 64$
$\Rightarrow 729{r^6} = 64$
$\Rightarrow {r^6} = {{64} \over {729}} = {\left( {{2 \over 3}} \right)^6}$
$\Rightarrow r = \frac { 2 } { 3 }$
$\Rightarrow S _ { n } = \frac { a \left( 1 - r ^ { n } \right) } { 1 - r }$ when r < 1
$\Rightarrow S _ { 7 } = \frac { 729 \left[ 1 - \left( \frac { 2 } { 3 } \right) ^ { 7 } \right] } { 1 - \frac { 2 } { 3 } } = \frac { 729 \left[ 1 - \frac { 128 } { 2187 } \right] } { \frac { 3 - 2 } { 3 } }$
$\Rightarrow S _ { 7 } = 729 \times 3 \left( \frac { 2187 - 128 } { 2187 } \right)$
$\Rightarrow \mathrm { S } _ { 7 } = \frac { 729 \times 3 \times 2059 } { 2187 } = 2059$ View full question & answer→Question 553 Marks
The Sum of first three terms of a G.P. is $16$ and the Sum of the next three term is $128$. determine the first term, the common ratio and the Sum to n terms of the G.P.
Answer$S_3=16$
$\frac{a\left(1-r^3\right)}{1-r}=16(1)$
$S_6-S_3=128$
$\frac{a\left(1-r^6\right)}{1-r}-16=128$
$\frac{a\left(1-r^6\right)}{1-r}=144(2)$
$(2) \div(1)$
$\frac{1-r^6}{1-r^3}=\frac{144}{16}$
$1+\mathrm{r}^3=9$
$\mathrm{r}^3=8$
$\mathrm{r}=2$
$s_3=\frac{a\left(r^3-1\right)}{r-1}=16$
$\mathrm{a}=16 / 7$
$s_n=\frac{a\left(r^n-1\right)}{r-1}=\frac{16}{7} \frac{\left(2^n-1\right)}{2-1}=\frac{16}{7}\left(2^n-1\right)$
View full question & answer→Question 563 Marks
How many terms of G.P. $3, 3^2 , 3^3$ ...... are needed to give the sum 120?
AnswerHere, a = 3 and r = $ \frac { 3 ^ { 2 } } { 3 } = 3$
$\therefore ^ { \mathrm { S } _ { n } = \frac { a \left( r ^ { - } - 1 \right) } { r - 1 } }$when r > 1
$\Rightarrow 120 = \frac { 3 \left( 3 ^ { n } - 1 \right) } { 3 - 1 }$
$\Rightarrow 120 = \frac { 3 } { 2 } \left( 3 ^ { n } - 1 \right)$
$ \Rightarrow 120 \times \frac { 2 } { 3 } = 3 ^ { n } - 1$
$ \Rightarrow 3^n = 81$
$\Rightarrow 3^n = (3)^4$
$\Rightarrow n = 4$
Therefore, the sum of 4 terms of the given G.P. is 120.
View full question & answer→Question 573 Marks
The sum of first three terms of a G.P. is $\frac{39}{10}$ and their product is 1. Find the common ratio and the terms.
AnswerLet $\frac ar$ , a, r be first three terms of the given G.P.According to question,$\frac { a } { l ^ { r } } + a + a r = \frac { 39 } { 10 }$……….(i)
And$\frac { a } { r } \times a \times r = 1$
$\Rightarrow a ^ { 3 } = 1$
$\Rightarrow a = 1$
Putting value of a in eq. (i),
$\Rightarrow 10+ 10r + 10r^2 = 39r$
$\Rightarrow 10r^2- 29r +10 = 0$
$\Rightarrow r = \frac { - ( - 29 ) \pm \sqrt { ( - 29 ) ^ { 2 } - 4 \times 10 \times 10 } } { 2 \times 10 }$
$\Rightarrow r = \frac { 29 \pm \sqrt { 841 - 400 } } { 20 }$
$\Rightarrow r = \frac { 29 \pm 21 } { 20 }$
Taking $r = \frac { 29 + 21 } { 20 } = \frac { 50 } { 120 } = \frac { 5 } { 2 }$ and
then the first three terms are $\frac { 1 } { 5 / 2 } , 1,1 \times \frac { 5 } { 2 }$
$\Rightarrow \frac { 2 } { 5 } , 1 , \frac { 5 } { 2 }$
Taking $r = \frac { 29 - 21 } { 20 } = \frac { 8 } { 20 } = \frac { 2 } { 5 }$
then first three terms are $\frac { 1 } { 2 / 5 } , 1,1 \times \frac { 2 } { 5 }$
$\Rightarrow\frac { 5 } { 2 } , 1 , \frac { 2 } { 5 }$
View full question & answer→Question 583 Marks
Evaluate:$\sum \limits_ { k = 1 } ^ { 11 } \left( 2 + 3 ^ { k } \right)$
AnswerGiven:$\sum _ { k = 1 } ^ { 11 } \left( 2 + 3 ^ { k } \right)$
$=\left(2+3^1\right)+\left(2+3^2\right)+\left(2+3^3\right)+\left(2+3^{11}\right)$
$=(2+2+2+\ldots \ldots . .11 \text { times })+\left(3+3^2+3^3+\ldots \ldots . .+3^{11}\right)$
$=22+\left(3+3^2+3^3+\ldots \ldots . .+3^{11}\right) \ldots \ldots \ldots . \text { (i) }$
Here $3,3^2, 3^3 \ldots \ldots . ., 3^{11}$ is in G.P.
$\therefore$a = 3 and r = $\frac { 3 ^ { 2 } } { 3 } = 3$
$\mathrm { S } _ { n } = \frac { 3 \left( 3 ^ { 11 } - 1 \right) } { 3 - 1 } = \frac { 3 } { 2 } \left( 3 ^ { 11 } - 1 \right)$
Putting the value of $S_n$ in eq. (i), we get $\sum _ { k = 1 } ^ { 11 } \left( 2 + 3 ^ { k } \right) = 22 + \frac { 3 } { 2 } \left( 3 ^ { 11 } - 1 \right)$
View full question & answer→Question 593 Marks
Find the sum to indicated number of terms of the geometric progression $x^3, x^5, x^7 ... n$ terms (if $x \ne \pm1)$.
AnswerHere,$a = x^3$ and r = $\frac { x ^ { 5 } } { x ^ { 3 } }$ = $x^2$
$S _ { n } = \frac { a \left( 1 - r ^ { n } \right) } { 1 - r }$ when r < 1
$\Rightarrow S _ { n } = \frac { x ^ { 3 } \left[ 1 - \left( x ^ { 2 } \right) ^ { n } \right] } { 1 - x ^ { 2 } }$
$\Rightarrow \mathrm { S } _ { n } = \frac { x ^ { 3 } } { 1 - x ^ { 2 } } \left[ 1 - x ^ { 2 n } \right]$
View full question & answer→Question 603 Marks
The sums of n terms of two arithmetic progressions are on the ratio 5n + 4:9n + 6. Find the ratio of their $18^{th}$ terms.
AnswerLet $a_1, a_2$ and $d_1, d_2$ be the first terms and common differences of two A.P’s respectively.$\therefore \frac { \frac { n } { 2 } \left[ 2 a _ { 1 } + ( n - 1 ) d _ { 1 } \right] } { \frac { n } { 2 } \left[ 2 a _ { 2 } + ( n - 1 ) d _ { 2 } \right] } = \frac { 5 n + 4 } { 9 n + 6 }$
$\Rightarrow \frac { 2 a _ { 1 } + ( n - 1 ) d _ { 1 } } { 2 a _ { 2 } + ( n - 1 ) d _ { 2 } } = \frac { 5 n + 4 } { 9 n + 6 }$
$\Rightarrow \frac { a _ { 1 } + \left( \frac { n - 1 } { 2 } \right) d _ { 1 } } { a _ { 2 } + \left( \frac { n - 1 } { 2 } \right) d _ { 2 } } = \frac { 5 n + 4 } { 9 n + 6 }$
Now, to get $18^{th}$ term, $\frac { n - 1 } { 2 } = 17$
$\Rightarrow$ n = 35
$\therefore \frac { a _ { 1 } + 17 d _ { 1 } } { a _ { 2 } + 17 d _ { 2 } } = \frac { 5 \times 35 + 4 } { 9 \times 35 + 6 }$
$\Rightarrow \frac { a _ { 1 } + 17 d _ { 1 } } { a _ { 2 } + 17 d _ { 2 } } = \frac { 179 } { 321 }$
Therefore, the ratio of $18^{th}$ terms of two A.P.’s is 179: 321.
View full question & answer→Question 613 Marks
If the sum of n terms of an A.P. is $(pn + qn^2)$, where p and q are constants, find the common difference.
AnswerGiven: $S_n=p n+q n^2$
Put $n=1$ we get, $S_1=p+p \Rightarrow a=p+q \ldots$... $(i)$
Now $S_n=p n+q n^2$
$\Rightarrow \frac{n}{2}[2 a+(n-1) d]=p n+q n^2$
$\Rightarrow \frac{n}{2}[2(p+q)+(n-1) d]=p n+q n^2 \text { (using.(i)) }$
$\Rightarrow 2 n(p+q)+n(n-1) d=2 p n+2 q n^2$
$\Rightarrow 2 n q+n(n-1) d=2 q n^2$
$\Rightarrow n(n-1) d=2 q n(n-1)$
$\Rightarrow d=2 q$
View full question & answer→Question 623 Marks
Find the sum of $n$ terms of an A.P. whose $k^{\text {th }}$ term is $5 k+1$.
AnswerGiven: $a_k=5 k+1$
Putting $\mathrm{k}=1$ and $\mathrm{k}=\mathrm{n}$, we get
$a=5 \times 1+1=6$ and $a_n=5 n+1$
$\therefore S_{n} = \frac{n}{2}(a + l)$
$\Rightarrow S_{n}=\frac{n}{2}(6+5 n+1)=\frac{n}{2}(5 n+7)$
View full question & answer→Question 633 Marks
If the sum of a certain number of terms of the A.P. $25, 22, 19$, ….. is $116$, find the last term.
AnswerHere $\mathrm{a}=25, \mathrm{~d}=22-25=-3$ and $\mathrm{S}_{\mathrm{n}}=116$
We have $S_n=\frac{n}{2}[2 a+(n-1) d]$
$\Rightarrow 116=\frac{n}{2}[2 \times 25+(n-1) \times(-3)]$
$\Rightarrow 232=n[50-3 n+3]$
$\Rightarrow 232=53 n-3 n^2$
$\Rightarrow 3 n^2-53 n+232=0$
$\Rightarrow$ n = $ \frac { - ( - 53 ) \pm \sqrt { ( - 53 ) ^ { 2 } - 4 \times 3 \times 232 } } { 2 \times 3 }$
$\Rightarrow$ n = $\frac { 53 \pm \sqrt { 2809 - 2784 } } { 6 }$
$\Rightarrow$ n = $\frac { 53 \pm \sqrt { 25 } } { 6 } = \frac { 53 \pm 5 } { 6 }$
$\Rightarrow$ n $\frac { 53 + 5 } { 6 }$ or n = $\frac { 53 - 5 } { 6 }$
$\Rightarrow$ n = $ \frac { 58 } { 6 }$ or n = $\frac { 48 } { 6 }$ = 8
But n = $ \frac { 58 } { 6 }$ is not possible as n $\in$ N. Therefore, n = 8
Now, $a_n = a + (n - 1)d$
$\Rightarrow a_8 = 25 + (8 - 1) \times (-3)$
View full question & answer→Question 643 Marks
In an A.P., if $p^{th}$ term is $\frac { 1 } { q }$ and $q^{th}$ term is $\frac { 1 } { p },$ prove that the sum of first pq terms is $\frac { 1 } { 2 } ( p q + 1 ),$ where $p \neq q.$
AnswerLet a be the first term and d be the common difference of given A.P.And $a _ { p } = \frac { 1 } { q }$ and $a _ { q } = \frac { 1 } { p }$
$\therefore a + ( p - 1 ) d = \frac { 1 } { q }$ and $a + ( q - 1 ) d = \frac { 1 } { p }$
$\Rightarrow a + p d - d = \frac { 1 } { q }$ …..(i) and $a + q d - d = \frac { 1 } { p }$…..(ii)
Subtracting eq. (ii) from eq. (i), we get
a + pd - d - (a + qd - d) $= \frac { 1 } { q } - \frac { 1 } { p }$
$\Rightarrow$ pd - d - a - qd + d $= \frac { p - q } { p q }$
$\Rightarrow ( p - q ) d = \frac { p - q } { p q }$
$\Rightarrow d = \frac { p - q } { p q } \times \frac { 1 } { p - q } = \frac { 1 } { p q }$
Putting value of d in eq. (i), we get
$a + p{1 \over {pq}} - d = {1 \over q}$
$\Rightarrow a + {1 \over q} - d = {1 \over q}$
$\Rightarrow a = {1 \over q} + d - {1 \over q} = d = {1 \over {pq}}$
Now, $S _ { n } = \frac { n } { 2 } [ 2 a + ( n - 1 ) d ]$
$\Rightarrow S _ { p q } = \frac { p q } { 2 } \left[ 2 \times \frac { 1 } { p q } + ( p q - 1 ) \times \frac { 1 } { p q } \right]$
$\Rightarrow S _ { p q } = \frac { p q } { 2 } \left[ \frac { 2 } { p q } + \frac { p q - 1 } { p q } \right]$
$\Rightarrow S _ { p q } = \frac { p q } { 2 } \left[ \frac { 2 + p q - 1 } { p q } \right]$
$\Rightarrow S _ { p q } = \frac { p q } { 2 } \left[ \frac { 1 + p q } { p q } \right] _ { = } \frac { p q + 1 } { 2 }$
$\Rightarrow S _ { p q } = \frac { 1 } { 2 } ( p q + 1 )$
View full question & answer→Question 653 Marks
How many terms of the A.P., $- 6 , \frac { - 11 } { 2 } , - 5 , \ldots$are needed to give the sum -25?
AnswerHere, a = -6, $d = \frac { - 11 } { 2 } - ( - 6 ) = \frac { - 11 } { 2 } + 6 = \frac { 1 } { 2 }$$\therefore {{\text{S}}_n} = \frac{n}{2}[2a + (n - 1)d]$
$\Rightarrow - 25 = \frac { n } { 2 } \left[ 2 \times ( - 6 ) + ( n - 1 ) \times \frac { 1 } { 2 } \right]$
$\Rightarrow - 50 = n \left[ - 12 + \frac { n - 1 } { 2 } \right]$
$\Rightarrow - 50 = n \left[ \frac { - 24 + n - 1 } { 2 } \right]$
$\Rightarrow-100=n^2-25 n$
$\Rightarrow n^2-25 n+100=0$
$\Rightarrow(n-20)(n-5)=0$
$\Rightarrow n=20 \text { or } n=5$
View full question & answer→Question 663 Marks
In an A.P. the first term is $2$ and the sum of the first five terms is one-fourth of the next five terms. Show that $20th$ term is $-112$.
AnswerAccording to question, $a=2$ and $\mathrm{S}_5=\frac{1}{4}\left[\mathrm{~S}_{10}-\mathrm{S}_5\right]$
$\Rightarrow 4 \mathrm{~S}_5=\mathrm{S}_{10}-\mathrm{S}_5$
$\Rightarrow 5 \mathrm{~S}_5=\mathrm{S}_{10}$
$\left.\Rightarrow \sqrt[5]{\left.\frac{5}{2}\{2 \times 2+(5-1) d\}\right]}=\frac{10}{2}[2 \times 2+(10-1) \mathrm{d}] \text { since } \mathrm{S}_{\mathrm{n}}=\frac{n}{2}[2 \mathrm{a}+(\mathrm{n}-1) \mathrm{d}\}\right]$
$\Rightarrow \frac{25}{2}[4+4 \mathrm{~d}]=5[4+9 \mathrm{~d}]$
$\Rightarrow 25[4+4 \mathrm{~d}]=10[4+9 \mathrm{~d}]$
$\Rightarrow 100+100 \mathrm{~d}=40+90 \mathrm{~d}$
$\Rightarrow 10 \mathrm{~d}=-60$
$\Rightarrow \mathrm{~d}=-6$
Now, $a_n=a+(n-1) d$
$\Rightarrow a_{20}=2+(20-1) \times(-6)$
$\Rightarrow a_{20}=2-114=-112$
View full question & answer→Question 673 Marks
Find the sum of all natural numbers lying between 100 and 1000 which are multiples of 5.
AnswerAccording to question, series is 105, 110, 115, 120, ………, 995
Here a = 105, d = 110 - 105 = 5 and $a_n = 995$
$\therefore a_n = a + (n - 1)d$
$\Rightarrow 995 = 105 + ( n - 1 ) \times 5$
$\Rightarrow 995 - 105 = ( n - 1 ) \times 5$
$\Rightarrow \frac { 890 } { 5 } = ( n - 1 )$
$\Rightarrow$ n = 178 + 1 = 179
Now, $\mathrm { S } _ { n } = \frac { n } { 2 } ( a + l )$
$\Rightarrow S _ { 199 } = \frac { 179 } { 2 } ( 105 + 995 )$
$\Rightarrow S _ { 179 } = \frac { 179 } { 2 } \times 1100=98450$
View full question & answer→Question 683 Marks
The difference between any two consecutive interior angles of a polygon is $5^{\circ}$. If the smallest angle is $120^{\circ}$. find the number of the sides of the polygon.
AnswerLet the number of sides of polygon be $n$. The interior angles of the polygon form an A.P.
Here, $a=120^{\circ}$ and $d=5^{\circ}$
Since, Sum of interior angles of a polygon with $n$ sides is $(n-2) \times 180^{\circ}$
$\therefore S_n=(n-2) \times 180^{\circ}$
$\Rightarrow \frac{n}{2}[2 \times 120+(n-1) \times 5]=180 n-360$
$\Rightarrow 120 n+\frac{5 n^2-5 n}{2}=180 n-360$
$\Rightarrow 240 n+5 n^2-5 n=360 n-720$
$\Rightarrow 5 n^2-125 n+720=0$
divide by 5 , we get
$\Rightarrow n^2-25 n+144=0$
$\Rightarrow(n-16)(n-9)=0$
$\Rightarrow n=16 \text { or } n=9$
But $\mathrm{n}=16$ not possible because $\mathrm{a}_{16}=\mathrm{a}+15 \mathrm{~d}=120+15 \times 5=195^{\circ}>180^{\circ}$
Therefore, number of sides of the polygon are 9
View full question & answer→Question 693 Marks
A man starts repaying a loan as first installment of ₹ 100. If he increases the installment by ₹ 5 every month, what amount he will pay in the $30^{th}$ installment?
AnswerAmount of $1^{st}$ installment = Rs. 100 and Amount of $2^{nd}$ installment = Rs. 105
The monthly installments 100,105,110, .............. form an A.P
$\therefore$ a = 100, d = 105 - 100 =5 and n = 30
Now $a_n = a + (n - 1)d$
$\Rightarrow a _ { 30 } = 100 + ( 30 - 1 ) \times 5$
$\Rightarrow a _ { 30 } = 100 + 29 \times 5$
$\Rightarrow a_{30} = 100 + 145 = Rs. 245$
Therefore, the amount of $30^{th}$ installment is Rs. 245.
View full question & answer→Question 703 Marks
Between 1 and $31, \mathrm{~m}$ numbers have been inserted in such a way that resulting sequence is an A.P. and the ratio of $7^{\text {th }}$ and $(m-1)^{\text {th }}$ numbers is $5: 9$. Find the value of $m$.
AnswerLet $A_1, A_2, A_3, A_4$, …….., $A_m$ be $m$ numbers between 1 and 31 . Here, $a=1$ and let the common difference be d .
$\therefore a_{m+2}=31$
$\Rightarrow a+(m+2-1) d=31$
$\Rightarrow 1+(m+1) d=31$
$\Rightarrow d=\frac{30}{m+1}$
Now, $\mathrm{A}_7=a+7 d=1+7 \times\left(\frac{30}{m+1}\right)=\frac{m+1+210}{m+1}=\frac{m+211}{m+1}$
And $\mathrm{A}_{\mathrm{m}-1}=\mathrm{a}+(\mathrm{m}-1) \mathrm{d}=1+(m-1) \times\left(\frac{30}{m+1}\right)=\frac{m+1+30 m-30}{m+1}=\frac{31 m-29}{m+1}$
According to question, $\frac{A_7}{A_{m-1}}=\frac{5}{9}$
$\Rightarrow \frac{\frac{m+211}{m+1}}{\frac{31 m+29}{m+1}}=\frac{5}{9}$
$\Rightarrow \frac{m+211}{31 m-29}=\frac{5}{9}$
$\Rightarrow 9 m+1899=155 m-145$
$\Rightarrow 146 m=2044$
$\Rightarrow m=14$
View full question & answer→Question 713 Marks
If the sum of $n$ terms of an A.P. is $3 n^2+5 n$ and its $m^{\text {th }}$ term is 164 , find the value of $m$.
AnswerGiven: $S_n=3 n^2+5 n$ and $a_m=164$
Put $\mathrm{n}=1$ and $\mathrm{n}=2$ in $\mathrm{S}_n$ we get,
$\mathrm{S}_1=3+5=8$
$\Rightarrow a=8 \ldots$
and $S_2=3(4)+5(2)=12+10=22$
We have $a_n=S_n-S_{n-1}$
$\therefore a_2=S_2-S_1$
$\Rightarrow a_2=22-8=14$
Now $d=a_2-a_1=14-8=6$
$\therefore a_m=164$
$\Rightarrow a+(m-1) d=164$
$\Rightarrow 8+(m-1) 6=164$
$\Rightarrow 8+6 m-6=164$
$\Rightarrow 6 m=162$
$\Rightarrow m=\frac{162}{6}=27$
View full question & answer→Question 723 Marks
The ratio of the sum of $m$ and $n$ terms of an A.P. is $m^2: n^2$ Show that the ratio of $m^{\text {th }}$ and $n^{\text {th }}$ term is $(2 m-1):(2 n-1)$.
AnswerLet a be the first term and d be the common difference of given A.P.
$\therefore S _ { m } = \frac { m } { 2 } [ 2 a + ( m - 1 ) d ]$ and $\mathrm { S } _ { n } = \frac { n } { 2 } [ 2 a + ( n - 1 ) d ]$
According to question, $\frac { S _ { m } } { S _ { n } } = \frac { \frac { m } { 2 } [ 2 a + ( m - 1 ) d ] } { \frac { n } { 2 } [ 2 a + ( n - 1 ) d ] } = \frac { m ^ { 2 } } { n ^ { 2 } }$
$\Rightarrow \frac { 2 a + ( m - 1 ) d } { 2 a + ( n - 1 ) d } = \frac { m ^ { 2 } } { n ^ { 2 } } \times \frac { n } { 2 } \times \frac { 2 } { m }$
$\Rightarrow \frac { 2 a + ( m - 1 ) d } { 2 a + ( n - 1 ) d } = \frac { m } { n }$
$\Rightarrow 2 a n+n(m-1) d=2 a m+m(n-1) d$
$\Rightarrow 2 a n-2 a m=(m n-m) d-(m n-n) d$
$\Rightarrow 2 a(n-m)=(m n-m-m n+n) d$
$\Rightarrow 2 a(n-m)=(n-m) d$
$\Rightarrow d=2 a$
Now, $\frac { a _ { m } } { a _ { n } } = \frac { a + ( m - 1 ) d } { a + ( n - 1 ) d } = \frac { a + ( m - 1 ) 2 a } { a + ( n - 1 ) 2 a }$
$= {{a(1 + 2m - 2)} \over {a(1 + 2n - 2)}} = {{2m - 1} \over {2n - 1}}$
$\therefore a_m:a_n = (2m - 1):(2n - 1)$
View full question & answer→Question 733 Marks
Sum of the first p, q and r terms of an A.P. are a, b and c respectively. Prove that $\frac { a } { p } ( q - r ) + \frac { b } { q } ( r - p ) + \frac { c } { r } ( p - q ) = 0.$
AnswerLet A be the first term and d be the common difference of given A.P. $\therefore S _ { p } = \frac { p } { 2 } [ 2 \mathrm { A } + ( p - 1 ) d ] = a$
$\Rightarrow A + \frac { p - 1 } { 2 } d = \frac { a } { p }$……….(i)
$\mathrm { S } _ { q } = \frac { q } { 2 } [ 2 \mathrm { A } + ( q - 1 ) d ] = b$
$\Rightarrow A + \frac { q - 1 } { 2 } d = \frac { b } { q }$……….(ii)
$\mathrm { S } _ { r } = \frac { r } { 2 } [ 2 \mathrm { A } + ( r - 1 ) d ] = c$
$\Rightarrow A + \frac { r - 1 } { 2 } d = \frac { c } { r }$……….(iii)
Now $\frac { a } { p } ( q - r ) + \frac { b } { q } ( r - p ) + \frac { c } { r } ( p - q ) = 0$
Putting the values of $\frac { a } { p } , \frac { b } { q }$ and $\frac { c } { r }$ from eq. (i), (ii) and (iii), we get
$\left[ \mathrm { A } + \frac { p - 1 } { 2 } d \right] ( q - r ) + \left[ \mathrm { A } + \frac { q - 1 } { 2 } d \right] ( r - p ) + \left[ \mathrm { A } + \frac { r - 1 } { 2 } d \right] ( p - q ) = 0$
$\Rightarrow A ( q - r + r - p + p - q ) + d \left[ \frac { p - 1 } { 2 } ( q - r ) + \frac { q - 1 } { 2 } ( r - p ) + \frac { r - 1 } { 2 } ( p - q ) \right] = 0$
$\Rightarrow A ( 0 ) + d \left[ \frac { p q - p r - q + r } { 2 } + \frac { q r - p q - r + p } { 2 } + \frac { p r - q r - p + q } { 2 } \right] = 0$
$\Rightarrow 0 + d \left[ \frac { p q - p r - q + r + q r - p q - r + p + p r - q r - p + q } { 2 } \right] = 0$
$\Rightarrow 0 + d \left[ \frac { 0 } { 2 } \right] = 0$
$\Rightarrow$ 0 = 0 = 0
$\Rightarrow$ 0 = 0
$\Rightarrow$ L.H.S. = R.H.S. Proved.
View full question & answer→Question 743 Marks
If the sum of first p terms of an A.P. is equal to the sum of the first q terms, then find the sum of the first $(p + q)$ terms.
AnswerLet a be the first term and d be the common difference of given A.P.
$\therefore$ $S _ { p } = \frac { p } { 2 } [ 2 a + ( p - 1 ) d ]$ and $\mathrm { S } _ { q } = \frac { q } { 2 } [ 2 a + ( q - 1 ) d ]$
According to question, $S_p = S_q$
$\Rightarrow \frac { p } { 2 } [ 2 a + ( p - 1 ) d ] = \frac { q } { 2 } [ 2 a + ( q - 1 ) d ]$
$\Rightarrow 2 a p+p^2 d-p d=2 a q+q^2 d-q d$
$\Rightarrow 2 a p-2 a q=q^2 d-p^2 d+p d-q d$
$\Rightarrow 2 a(p-q)=\left[-\left(p^2-q^2\right) d+(p-q) d\right]$
$\Rightarrow 2 a(p-q)=[-(p-q)(p+q) d+(p-q) d]$
$\Rightarrow 2 a(p-q)=(p-q)[1-p-q] d$
$ \Rightarrow$ a = $\frac { ( 1 - p - q ) d } { 2 }$
Now $\mathrm { S } _ { p + q } = \frac { p + q } { 2 } \left[ \frac { 2 ( 1 - p - q ) d } { 2 } + ( p + q - 1 ) d \right]$
$= \frac { p + q } { 2 } [ d - p d - q d + p d + q d - d ]$
$\Rightarrow S _ { p + q } = \frac { p + q } { 2 } \times 0 = 0$
View full question & answer→Question 753 Marks
Find the sum of odd integers from 1 to 2001.
AnswerOdd integers from 1 to 2001 are $1,3,5,7, \ldots, 2001$.
Here, $a=1, d=3-1=2$ and $a_n=2001$
$\therefore a_n=a+(n-1) d$
$\therefore 2001=1+(n-1) \times 2$
$\Rightarrow 2001-1=(n-1) \times 2$
$\Rightarrow \frac{2000}{2}=(n-1)$
$\Rightarrow n=1000+1=1001$
Now, $\mathrm{S}_{\mathrm{n}}=\frac{n}{2}(\mathrm{a}+\mathrm{I})$
$\Rightarrow S_{1001}=\frac{1001}{2}(1+2001)$
$\Rightarrow S_{1001}=\frac{1001}{2} \times 2002=1002001$
View full question & answer→